68 Instinct and Intelligence 



tion of nervous matter was effected, forming 



an organ which we recognise as the animal's 



brain. 



The body of a large earth-worm contains a 



series of from 100 to 200 segments or somites, 



each of which is fur- 

 nished with four pairs of 

 bristles or chaetae mov- 

 able by muscles, and con- 

 stituting the animal's 

 chief locomotory organs. 

 An earth-worm's mouth 

 is situated on the under 

 surface of its head; at 

 the posterior extremity 

 of its body is a terminal 

 opening or anus. The 

 nervous system of these 



medio-dorsal nerve tracts. worms consists of tWO 



masses of nerve cells and fibres (ganglia) 

 situated just above the passage leading from the 

 animal's mouth to his stomach (Figs. 9 and 10). 

 These ganglia give rise to the nerves passing 

 forwards to the head and to a ganglion situated 

 below the gullet, from whence a cord of 



FIG. 9. a, cerebral ganglia ; 

 b, mouth ; c, ventral nerve 



